A steel caster is an apparatus for carrying out continuous casting, also referred to in the art as strand casting. Continuous casting involves transferring molten steel from a steelmaking furnace into a ladle. From the ladle, the molten steel is fed through a shroud of the ladle (also referred to as a ladle shroud) extending into a container or vessel referred to as a tundish. The molten steel typically is fed at a continuous or semi-continuous liquid flow into a molten steel bath contained in the tundish. The tundish typically acts as a reservoir from which the molten steel may be fed, without interruption or unwanted downtime, into caster molds. In order to protect the molten steel in the tundish from unwanted chemical reaction, e.g., excessive oxidation, and air-borne particulates, a protective slag cover/layer or “flux” is allowed to form at the surface of the molten steel bath.
Surface requirements and cleanliness standards of modern high quality steel products allow for very low tolerances of impurities and chemical changes. Impurities and chemical changes often are the result of turbulence created by the incoming ladle stream of molten steel fed into the tundish. Certain tundish designs for receiving liquid steel from the ladle shroud lead to unfavorable fluid flow conditions, such as turbulence, inside the tundish and promote high free surface flow activities. For example, the fluid flow generated by an incoming ladle stream may be reflected from the flat tundish floor and sidewalls toward the surface of the liquid steel. This generated fluid flow causes a turbulent boiling action, extensive wave motion, and splashing at the surface of the steel bath.
For example, FIG. 10 illustrates a longitudinal cross section of a single strand tundish 1 having an asymmetrical fluid flow 9a. The ladle shroud 7 is shown adjacent end wall 3 opposite a well block (not shown in FIGS. 10 and 11). Water flow-model studies have shown that the fluid flow, generated by an incoming ladle stream 8 from the ladle shroud 7, is reflected from the flat tundish floor 4 in an upward direction toward the surface of the liquid steel. If this fluid flow is restricted by the tundish walls 2 and 3, the restricted fluid flow is forced upward along the surface of such walls 2, 3. This upward flow follows a circular path 9c, and creates an upward surge along the face of the end wall 3, and a downward flow around a ladle shroud 7. The upward surge of the circular flow 9c causes excessive turbulence at the surface of the bath. These high free surface activities in the tundish give rise to a phenomenon called “open-eye,” whereby the protective slag cover 6 on top of the steel bath is broken. The broken slag cover 6 exposes the liquid steel to the surrounding atmosphere and sets up conditions conducive to altering the chemistry of the steel bath and creating inclusions in the steel bath. The chemical changes typically involve loss of aluminum from the bath and/or absorption of oxygen and nitrogen into the steel bath and consequent surface re-oxidation. Re-oxidation and other undesired reactions can introduce, for example, excess alumina, manganese sulfide, and calcium sulfide into the steel bath. Additionally, the downward flow around the ladle shroud 7 generates shear and vortices, and entraps and pulls broken particles 10 from the broken flux cover 6 down into the liquid steel bath. These broken particles 10 eventually are discharged from the tundish with the molten steel and create inclusions within the finished steel product.
The chemical changes and inclusions ultimately reduce steel quality and are a primary cause of rejection of high value steel grades such as HIC and armor plate grades. Further, splashing of the high temperature liquid steel against the tundish walls may present safety hazards for operators. Using conventional equipment, problems can also arise with respect to lack of steel bath temperature homogeneity and insufficient residence time to allow inclusion particles to float to the protective slag cover, where the particles can be isolated and/or separated from the liquid steel.
There have been various attempts to reduce or eliminate surface turbulence within a continuous caster tundish to improve the quality of the finished steel product. These attempts have included a wide assortment of dams and weirs which redirect the ladle stream fluid flow away from the surface of the molten steel bath. Although some known fluid flow control devices have been somewhat successful in controlling fluid flow and reducing surface turbulence, such control devices tend to be insufficient for the demands of high quality steel and/or cause operational problems such as those described above.